A common problem when connecting networks is that of ensuring messages from one network are correctly routed to a destination on the other network. A particular example of this is in the connection of networks using Internet Protocol (IP) to the existing telephone switched circuit network (PSTN). The infrastructure uses very different technologies and protocol: for example a PSTN conventionally uses the SS7 protocol. It is desirable, where two such networks are linked, to reliably provide PSTN signalling over the IP network.
An approach to providing a reliable connection between IP and SS7 networks has been proposed by the Internet Engineering Task Force (IETF). This includes M3UA, which is a protocol for the transport of any MTP3 signalling over IP, and SUA which defines the protocol for the transport of any SCCP User Part signalling over IP. Two main entities are defined: the signalling gateway (“SG”), which is the connection point between the SS7 and IP networks, and the application server (AS) which is the software application provided on the IP network which it is desired to make available over the SS7 network. The protocols may be used to connect SS7-based signalling end points (SEP) with an IP based AS thus allowing SS7 networks to access IP based applications.
A function performed by the signalling gateway is to receive SS7 signalling messages and direct them to the appropriate AS. To do so, the SG identifies a routing key (RK) defined by the M3UA or SUA protocol, which uniquely identifies the AS in accordance with parameters in the SS7 message. In general, the RK parameters are found in the header of the SS7 message, although it may be desirable to take into account other elements of the message, such as a SMS originating number or even some aspect of the message content, in order to direct the message to the appropriate AS.
A problem with using the M3UA or SUA protocol arises when it is desired to migrate an application from a legacy entity on the SS7 network to an AS on the IP network. As the M3UA and SUA protocols only allow the matching of an application server to the parameters of an SS7 message, it is necessary to have a complete transition at one step between the legacy SS7 application and the new IP application provided on an AS. The protocols do not allow a smooth transition in which traffic can be shared between the legacy application on the SS7 network and the new application on the IP network.